What is the molarity of a solution containing 10 mg of CaCl2 made with 100 mL of distilled water? Assume a gram molecular weight from the periodic table of approximately 111 g/mol.

• A. 9 × 10-4
• B. 1.1 × 10-3
• C. 11.1
• D. 90

Answer: (A)

Molarity is moles of solute per liter of solution. Convert mass to moles using the given molar mass, then divide by the volume in liters.

10 mg is 0.010 g. Moles of CaCl2 = 0.010 g ÷ 111 g/mol ≈ 9.0 × 10^-5 mol. The volume is 100 mL = 0.100 L. Molarity = (9.0 × 10^-5 mol) ÷ 0.100 L = 9.0 × 10^-4 M.

So the concentration is about 9 × 10^-4 M, matching the given answer.